Nanoparticle Formation Kinetics and Mechanistic Studies Important to Mechanism-Based Particle-Size Control: Evidence for Ligand-Based Slowing of the Autocatalytic Surface Growth Step Plus Postulated Mechanisms

2019-06-06
Ligands are known to affect the formation, stabilization, size, and size-dispersion control of transition-metal and other nanoparticles, yet the kinetic and mechanistic basis for such ligand effects remains to be elucidated and then coupled to predictions for improved particle size and narrower particle size distribution syntheses. Toward this broad goal, the effect of the added excess ligand (L) and the stabilizer, L = POM9- (= the polyoxometalate, P2W15Nb3O629-) is studied for the formation of POM9--stabilized Ir(0)(n) nanoparticles, {Ir(0)(n)center dot(POM9-)(m)}(9m-), synthesized from an atomically characterized precatalyst (COD)Ir center dot POM8- under H-2. First, the balanced reaction stoichiometry and characterization of the nanoparticle products are established. Next, the kinetics of nanoparticle formation is analyzed initially by the FW 2-step minimum mechanism consisting of slow, continuous nucleation, A -> B (rate constant k(1obs)), and autocatalytic surface growth, A + B -> 2B (rate constant k(2obs)) where A is nominally (COD)Ir center dot POM8- and B is nominally the growing, average {Ir(0)(n)center dot(POM9-)(m)}(9m-) nanoparticle. The autocatalytic surface growth rate constant, k(2obs), was then studied as a function of the amount of added POW9-. An inverse, quadratic-root-type dependence of k(2obs), on the concentration of L = POM9- is observed, which was then analyzed in terms of two main mechanisms. Specifically, the dependence of k(2obs), on the [POM9-] was analyzed in terms of (i) an A center dot L (sic) A + L dissociative equilibrium, (COD)Ir-I center dot POM8- + 2 solvent (sic) (COD)Ir-I(solv)(2)(+) + POM9-, and then (ii) this same A center dot L (sic) A + L plus also a B + L (sic) B center dot L nanoparticle-surface capping equilibrium, where B represents the average Ir(0)(n) nanoparticle. Three other mechanisms were also considered. The high-resolution transmission electron microscopy of the parent nanoparticles when no excess POM9- has been added is also provided as part of the Supporting Information. The results (a) provide the first evidence and resultant physical insight, for the prototype, well-studied {Ir(0)(n)center dot(POM9-)(m)}(9m-) nanoparticle formation system, that growth is a function of the amount of POM9- ligand present and (b) provide compelling evidence that A = (COD)Ir-I(solv)(2)(+) from the A center dot L dissociative equilibrium, (COD)Ir-I center dot POM8- + 2 solvent = (COD)Ir-I(solv)(2)(+) + POM9-, is the actual reactant in the FW 2-step formulation of the A + B -> 2B autocatalytic growth step. The results also (c) support the 1-step more complex mechanism that adds a ligand-capping B + L (sic) B center dot L step, namely, the mechanism consisting of the steps of A center dot L (sic) A + L, then A -> B, then A + B -> 2B, and then B + L (sic )B center dot L. Given the wide usage of the simpler FW 2-step mechanism, plus the fact that nanoparticle-stabilizing and -capping ligands are invariably present, one can anticipate a much broader applicability of mechanisms containing the A center dot L (sic) A + L and the B + L (sic) B center dot L steps to nanoparticle formation reactions.
JOURNAL OF PHYSICAL CHEMISTRY C

Suggestions

Metal-Enhanced Fluorescence from Silver Nanowires with High Aspect Ratio on Glass Slides for Biosensing Applications
Abel, Biebele; Coskun, Sahin; Mohammed, Muzaffer; Williams, Richard; Ünalan, Hüsnü Emrah; Aslan, Kadir (American Chemical Society (ACS), 2015-01-08)
High enhancement of fluorescence emission, improved fluorophore photostability, and significant reduction of fluorescence lifetimes have been obtained from high aspect ratio (>100) silver (Ag) nanowires. These quantities are found to depend on the surface loading of Ag nanowires on glass slides, where the enhancement of fluorescence emission increases with the density of nanowires. The surface loading dependence was attributed to the creation of intense electric fields around the network of Ag nanowires and...
Magnetically separable rhodium nanoparticles as catalysts for releasing hydrogen from the hydrolysis of ammonia borane
Tonbul, Yalcin; Akbayrak, Serdar; Özkar, Saim (Elsevier BV, 2019-10-01)
Magnetically separable catalysts attract considerable attention in catalysis due to their facile separation from the reaction medium. This propensity is crucial for efficient multiple use of precious noble metal nanoparticles in catalysis. In fact, the isolation of catalysts from the reaction medium by filtration and washing results usually in the loss of huge amount of activity in the subsequent run of catalysis. Although many transition metal nanoparticle catalysts have been reported for the H-2 generatio...
Conjugation effects on carrier mobilities of polythiophenes probed by time-resolved terahertz spectroscopy
Esentürk, Okan; Deongchamp, Dean M.; Heilweil, Edwin J. (American Chemical Society (ACS), 2008-07-24)
Optically generated carrier conductivity of thiophene-based spin-cast polymer films, as cast poly(3-hexylthiophene) (P3HT) and both as cast and above liquid crystalline temperature annealed poly(2,5-bis(3tetradecylthiophen-2yl)thieno[3,2-b]thiophene) (PBTTT), were measured by time-resolved THz spectroscopy (TRTS) and compared to reported thin-film transistor (TFT) device measurements. The relative mobilities of the samples measured by TRTS agree well with reported TFl` mobilities. Since TRTS is sensitive to...
Understanding the Dielectric Properties of Heat-Treated Carbon Nanofibers at Terahertz Frequencies: a New Perspective on the Catalytic Activity of Structured Carbonaceous Materials
Parrott, Edward P. J.; Zeitler, J. Axel; McGregor, James; Oei, Shu-Pei; Ünalan, Hüsnü Emrah; Tan, Swee-Ching; Milne, William I.; Tessonnier, Jean-Philippe; Schloegl, Robert; Gladden, Lynn F. (American Chemical Society (ACS), 2009-06-18)
Terahertz time domain spectroscopy (THz-TDS) has been used to study the electrical and optical properties of a series of carbon nanofibers (CNFs) that have undergone different heat treatments. The high-temperature heat-treated (HHT) sample displayed increased absorption and real refractive indices across the range 0.3-3.5 THz when compared to the low-temperature heat-treated (LHT) and pyrolitically stripped (PS) samples. The experimental results were fitted by using a Drude-Lorentz model and an effective me...
Molecular-dynamics simulations of surface and bulk properties of Zn, Cd, and ZnCd systems
Amirouche, L; Erkoç, Şakir (Wiley, 2004-02-01)
Surface and bulk properties of Zn, Cd, and ZnCd systems have been investigated by performing molecular-dynamics simulations using a recently developed empirical many-body potential energy function for these systems, which comprices two- and three-body atomic interactions. Surface reconstruction and multilayer relaxation on clean surfaces, adatom on surface, substitutional atom on surface and bulk materials, and vacancy on surface and bulk materials have been studied extensively. (C) 2004 WILEY-VCH Verlag Gm...
Citation Formats
S. Özkar, “Nanoparticle Formation Kinetics and Mechanistic Studies Important to Mechanism-Based Particle-Size Control: Evidence for Ligand-Based Slowing of the Autocatalytic Surface Growth Step Plus Postulated Mechanisms,” JOURNAL OF PHYSICAL CHEMISTRY C, pp. 14047–14057, 2019, Accessed: 00, 2020. [Online]. Available: https://hdl.handle.net/11511/41168.